Product and kinetic studies have been made on the pyrolysis of a series of 3-cyano-3-carbomethoxy-1-pyrazolines substituted at C-4 with a methyl and an aryl group (phenyl, p-methoxyphenyl, and p-nitrophenyl). Evidence is presented supporting a transition state structure in which little progress to bond breaking of the C-5 to N bond has taken place at the transition state. The product distribution is largely dependent on the stereochemistry of the initial pyrazoline and to a lesser degree on the nature of the aryl group at C-4. The cis pyrazolines gave products resulting primarily from aryl migration while the trans pyrazolines gave products predominantly from methyl migration.